Earlier, the Chile international posted a video on Instagram, apparently showing him walking across an airstrip towards a private jet following reports that he was due in north-west England for a medical.

Britain’s Daily Telegraph newspaper said Sanchez had been pictured arriving at United’s Carrington training complex to complete his move from the London club, widely expected to be finalised on Monday.

Sky reported that Mkhitaryan would have a medical in London on Sunday.

Chile’s Sanchez, 29, is set to become United’s best-paid player, with some reports putting his salary at £400 000 per week, prompting a warning from former England boss Steve McClaren that his high wages could create dressing-room disharmony.

“Wage parity is very important to players. Yes, they find out – at all levels – and if someone is paid a lot higher than others that can cause a problem,” he said.

Arsenal boss Arsene Wenger and United manager Jose Mourinho both said they expected the two-way deal between the Premier League rivals to go through.

Wenger said Sanchez was absent from the Arsenal squad on Saturday because he was close to completing a deal that has been in the making since United outbid Manchester City for the former Barcelona star.

“I expect soon or never,” Mourinho said after his side’s 1-0 win against Burnley. “I think it is so close, so close, so close that if it doesn’t happen, it’s not going to happen.”

Sky said it understood it could be a straight swap deal, but details were yet to be finalised.

McClaren, who was Alex Ferguson’s assistant when Manchester United won the treble in 1999, said the Sanchez signing would be a major coup for the club, but warned his earnings could unsettle other players.

“If he (Sanchez) is on the wages that is being bandied about, then other players will know that when their contracts come up or they are in demand from other clubs,” he said.

But McClaren believes United have struck a major blow by landing Sanchez, who has scored 80 goals in 166 appearances at Arsenal.

“Any manager worth his salt would love to have Sanchez in their team,” McClaren told BBC Radio 5 Live.

“Sanchez coming in for the second half of the season at Manchester United is a huge boost to them.

“He has got that experience and toughness, he wins games and he is a leader. It will lift the level of the whole club.”